JPH02136813A - Image pickup device - Google Patents

Abstract

PURPOSE:To prevent the occurrence of moire without spoiling resolution by providing 1st and 2nd optical low-pass filters respectively having specific spatial frequency responses on the image forming optical path from the emitting end face of an image guide fiber bundle to a solid-state image pickup element. CONSTITUTION:The 1st optical low-pass filter F1 having a spatial frequency response corresponding to the fiber array of an image guide fiber bundle 1 and the 2nd optical low-pass filter F2 having a spatial frequency response corresponding to the sampling frequency of a solid-state image pickup element 3 are provided on the image forming optical path from the emitting end face of the fiber bundle 1 to the element 3. Therefore, the occurrence of moire can be prevented without spoiling the resolution when observations are made with a TV monitor by selectively fitting cameras of the same constitution equipped with an image forming optical system with variable or fixed image forming magnifications to plural fiberscopes having image guide fiber bundles 1 of different effective diameters and core diameters.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a suspended image device for capturing an image appearing on the east end surface of an image guide fiber of an optical instrument such as a fiberscope equipped with an image guide fiber bundle. .

[Conventional technology]

Recently, 'T' V force has been applied to the eyepiece of fiberscopes, etc.!
It has become popular to observe the inside of the body cavity using a TV set.

Incidentally, the end face of an image guide fiber bundle used in a fiberscope has regular bright and dark patterns (mesh structure Ja) determined by the arrangement of the fiber cores. On the other hand, if a camera uses an image pickup tube and has a color stripe filter on the curve of its light-receiving surface, or if it uses a solid-state image sensor and has a color mosaic filter in front of it, then The arrangement of each color element of the filter has a regular arrangement, and even in the case where there is no mosaic filter, the picture elements of the solid-state image sensor have a regular arrangement. Therefore, there is a problem in that both regular structures interfere with each other, causing moiré in the TV image.

Therefore, as a method for removing this type of moire, it is known to provide an optical low-pass filter between the end face of the image guide fiber bundle and the solid-state image sensor. for example,
The device described in Japanese Unexamined Patent Publication No. 55-143125 uses a phase filter as an optical low-pass filter.

Also, what is described in Japanese Patent Application Laid-open No. 59-163416 is
A birefringent filter combined with a quartz plate is used as an optical low-pass filter.

[Problem to be solved by the invention]

However, in recent years, the number of types of fiberscopes has increased, and for example, fiberscopes are used to view the inside of blood vessels, etc.
There are various types of endoscopes, ranging from those with a diameter of about 5 as to quite thick ones for viewing the inside of the large intestine, etc., and industrial endoscopes that are even thicker. When observing these images using the same TV camera, it is necessary to ensure that the image size is approximately the same no matter which scope is used, so the end face of the image guide fiber bundle is connected onto the solid-state image sensor. The magnification when imaging differs significantly from scope to scope, and as a result, the thickness of the core in the image (that is, on the imaging surface) also differs significantly.If we talk about this in the frequency domain, it is as follows. The solid-state+i image element samples an object image at a spatial frequency corresponding to the repetition period of the regular structure. As is known from communication theory, if the frequency spectrum band of the sampled signal extends to a high frequency higher than the Nyquist frequency, so-called aliasing distortion occurs, which becomes a cause of moiré.

In this case, the sampled signal is an image of the end face of the image guide Noah 9 bundle, and its spatial frequency spectrum is
If the brightness/darkness change due to the repetition of the core is a sine wave, it will be up to this repetition frequency. In reality, the brightness change is not a sine wave, so harmonic components also exist, but the fundamental wave component is the largest in terms of spectrum size.

Therefore, it can be considered that the bandwidth of the spatial frequency spectrum of the image is substantially determined by the repetition period of the core in the image.

This means that the manner in which moiré occurs is greatly influenced by the thickness of the core in the image of the east end face of the image guide fiber on the imaging element.

Since the magnitude of the spatial frequency spectrum component determined by the period of the core is very large, this component is the main cause of moiré.

The relationship between the Nyquist frequency of the solid-state image sensor and the core spatial frequency of the image of the east end face of the image guide fiber is considered to have the shapes shown in FIGS. 1θ to 12.

These figures represent a two-dimensional spatial frequency plane,
f□ is an axis showing the frequency in the horizontal scanning direction, and rv is the axis showing the frequency in the vertical scanning direction. The horizontal and vertical Nyquist frequencies of the solid-state image sensor are fNN and rVH, respectively. Also, among the fibers used in the fiberscope, the diameter of the thinnest fiber is φ, and the diameter of the thicker fiber is φ, and the minimum value of the magnification when the end face of the image guide fiber bundle is imaged on the solid-state image sensor is β, and the maximum value is β', the thickness of the core in the image on the solid-state imaging device is a value within the range of φβ to φ゛β'. Therefore, the spatial frequency of the core is distributed in the range of 1/φ゛β゛ to 1/φβ.

In the case of FIG. 13, since the core frequency is less than the Nyquist frequency (that is, the core is imaged thickly), moiré is produced. Therefore, the cases of FIG. 1θ and FIG. 11 should be considered.

In the case shown in Figure 10, the spatial frequency of the core is higher than the Nyquist frequency of the solid-state image sensor, so if an optical low-pass filter is installed to remove the spatial frequency component of the core, the occurrence of moiré will be extremely small (virtually negligible). become. In terms of appearance, the gap between the cores is filled so that the network structure of one core becomes invisible by blurring the image using an optical low-pass filter or by forming multiple images that are slightly shifted. Furthermore, even in this case, since the spatial frequency components are removed outside the Nyquist frequency of the solid-state 111 imaging element, there is no significant effect on the resolution of the TV image that is best shadowed.

In the case of FIG. 11, the Nyquist frequency is within the core spatial frequency distribution band. This is a problem because it causes That is,
In the vertical direction, as in the case of Fig. 10, it is good because an optical low-pass filter is provided so that the response is zero in the spatial frequency distribution band of 17, but in the horizontal direction, The response drops on the inner side (lower frequencies), causing a drop in resolution.

In view of the problem mentioned in item -11, one object of the present invention is to provide an imaging device that can prevent the occurrence of moiré even in such a case without lowering the resolution to II.

[Means and effects for solving the problem] The imaging device according to the third aspect of the invention includes an objective lens/aperture that forms an image of an object, and an object lens that forms an image of an object;
an image guide fiber bundle that transmits the object image by the lens, and an imaging lens that reimages the object image appearing on the exit end face of the east fiber into a solid-state general image element f-1-. 13, the image guide fiber bundle If) - Noah, Ih j, j, . . .
, a first optical low-pass filter having a spatial frequency response corresponding to the sampling frequency of the solid-state image sensor, and a second optical low-pass filter having a spatial frequency response corresponding to the sampling frequency of the solid-state image sensor. It is characterized by being provided in the imaging optical path from the end face to the solid-state image sensor. This basic idea is explained in the 11th
This will be explained using the example shown in the figure.

In FIG. 11, in the vertical direction, the core spatial frequency 1ξ, which causes moiré, is J.Ikis! of the solid-state image sensor!・Distributed in a region higher than the frequency.

Therefore, if the response of the imaging optical system is made extremely small in this frequency range using the first optical low-pass filter, almost no moire will appear, and the resolution of both TV images will be reduced. '1. No.

On the other hand, in the horizontal direction, if the same method is used in the 1F direct force direction, the response will be reduced to a frequency range that does not cause moiré at the Nyquist frequency. Therefore, by providing a second optical low-pass filter whose response is extremely small at the Nyquist frequency of the solid-state image sensor, it is possible to prevent the occurrence of moiré without significantly reducing resolution.

Also, even in vertical and b directions, moiré can be removed by lowering the response below the Nyquist frequency, but in order not to reduce the resolution, it is better to keep the frequency as high as possible and keep it at a large value of L-λ''+47. Therefore, if the cause of the air emission is clear, such as the image of the image of the east end of the image guide fiber, the spectrum in the object image is Removing the components is more advantageous in terms of maintaining image quality.

Thus, a first optical low-pass filter, l!, with a spatial frequency toss response corresponding to the image-guiding fiber bundle, f; By providing a second optical low-pass filter having a spatial frequency response corresponding to the image element &11 in the imaging optical path, both moiré removal and resolution maintenance can be achieved.

In order to simplify the principle explanation, the Nyquist frequency wavenumber of the solid-state image sensor is set by one each in the horizontal and vertical directions. The frequencies may be different, and depending on how the amber elements of the color mosaic filter are arranged, the sampling frequency may be diagonal, so the end point of the image guide fiber bundle is the rule that people in solid-state imaging devices have. D-
It is necessary to determine the spatial frequency characteristics of the bus filter.

(Example) Hereinafter, the present invention will be described in detail based on the illustrated embodiments.

FIG. 1 shows a first embodiment in which the fiber arrangement is irregular in the image guide fiber and one image magnification is variable. 1 is an image guide fiber bundle, 2 is an imaging lens which is a zoom lens arranged on the exit side of the image guide fiber bundle 1, and 3 is a solid-state imaging device arranged on the imaging plane G of the imaging lens 2. F,
is the first optical lens 11 arranged on the incident side of the imaging lens 2.
- Bass filter, FT: is a second optical low-pass filter arranged on the exit side of the imaging lens 2, and its stiffness is composed of a crystal plate 4.5 or 6.

The first 1/lens of the imaging lens 2 is an eyepiece, and the optical system on the front side thereof is placed inside the fiberscope 7, and the optical system on the rear side is placed inside the TV left camera.

The spatial frequency response (MTF) of a single crystal plate normally has a characteristic of <1 cos θ1 as shown in Figure 2 (A), but this is only a - cross section, that is, the response is perpendicular to the plane of the paper. If the spatial frequency is expressed in two dimensions as shown in Fig. 2 (B), the frequencies at which the response becomes zero are on a straight line. The intersection of this line (referred to as a trap line) and the horizontal frequency axis is f, and the cross-sectional view taken along a plane perpendicular to the plane of the paper containing rH is shown in FIG. 2(A).

Figure 3 shows the spatial frequency response on the image plane using the same drawing method as in Figure 2 (B), and shows the inner ring band 0. is the range in which the spatial frequency of the core of the image guide fiber bundle l exists on the telephoto side of the zoom lens, and the outer ring-shaped band Ow is the range in which the spatial frequency of the core exists on the wide-angle side. The image is larger on the tele side (
Since the core is projected thicker, the spatial frequency of the core naturally becomes lower. The innermost rectangle is a line connecting the horizontal and vertical Nyquist frequencies. The situation on the telephoto side is exactly the same as that shown in Figure 11.

The thickness of the crystal plate 4°5 of the first optical low-pass filter F is appropriately determined so that the line where the response becomes zero in the vertical direction, that is, the trap line, is exactly at the outer and inner ends of the core spatial frequency band 07. (Create trap lines 4a and 4b with one crystal plate 4, and trap lines 5a and 5b with the other crystal plate 5.)

In the horizontal direction, the thickness of the crystal plate 6 of the second optical low-pass filter Ft is appropriately determined so that the trap lines 5a and 5b have the Nyquist frequency f. +'HN and parallel to the vertical axis. The concrete structure of the first optical low-pass filter F is as shown in FIG. It is configured. The incident light is split into two linearly polarized lights (ordinary light and extraordinary light) by the crystal Fi4. and 1
The /4λ plate 9 converts each beam into circularly polarized light, and the crystal plate 5 separates each beam into two linearly polarized beams. Since the amount of separation is smaller in the crystal plate 4, trap cottons 4a and 4b with high frequency are given.
Since the crystal plate 5 has a larger separation amount, it provides trap lines 5a and 5b with lower frequencies.

Since the crystal plate 4.5 is located on the incident side of the imaging lens 2, when the imaging magnification changes due to zooming, the amount of light beam separation also changes. Therefore, the relationship between the core frequency band and the trap line remains unchanged even when zooming. That is, the trap lines 4a, 5a, etc. on the telephoto side automatically become traps 94a', 5', etc. on the wide side, passing through the outer and inner ends of the core frequency band 0.1.

The second optical low-pass filter Ft is composed of one crystal plate 6
Since this has nothing to do with zooming, the trap lines 6a and 6b are always at the Nyquist frequency 'H
N+ f Passes through MN.

Thus, according to this embodiment, moiré can be effectively removed without reducing the resolution, but when wide, the trap lines 6a and 6b are completely away from the core spatial frequency band 0° in the horizontal direction, so it is difficult to remove moiré. The effect is extremely weak, so it is suitable for use mainly on the telephoto side.0 For example,
When capturing the image of an ultrafine endoscope such as an angioscope with a TV camera equipped with a zoom lens, only the magnification side (telephoto side) is used because the image guide fiber bundle is thin.

Therefore, this embodiment is suitable for such a device.

Up to now, it has been explained that the spatial frequency of the core of the plurality of image guide fiber bundles is somewhere in a ring-shaped frequency band. However, in recent years, image guide fiber bundles with less regular fiber arrangement have come into use, and in these types of image guide fiber bundles, one image guide fiber bundle has various frequency components, so the spatial frequency of the core may vary. may extend over the entire ring-shaped frequency band.This embodiment is also effective for such cases.

In addition, in FIG. 1, the first lens of the imaging lens 2 is an eyepiece, and the first optical low-pass filter F1 is located inside the fiberscope, but the filter F
, , F, and lens 2 may all be located within the camera, with only the eastern part of the image guide fiber being removable.

FIG. 5 shows the second embodiment, which uses two fiberscopes with irregular fiber arrangement and different effective diameters, each having a 1" Noah-Iha bundle, with a fixed imaging magnification and mount. Same 1; It is designed to be used by selectively attaching 1 ■V to a TV camera. 11 is an image guide fiber bundle with a small Tr efficiency and core diameter, and ■2 is an image guide fiber bundle with h effect IY and a core diameter of 1. 13 is an imaging lens with a fixed magnification, and 14 is a solid-state imaging device disposed on the imaging surface of the imaging lens 2. F'+, Fl' are the first optical []-pass filters, F, where the ray bone Ml is placed between the image guide fiber 111.12 and the imaging lens 13, respectively; A second optical 17-pass filter disposed between the imaging lens 13 and the solid-state image sensor 14, the first optical low-pass filter F
, ,F,' are respectively composed of two crystal plates 15 and 16, and the second optical low-pass filter FX is shown in FIG. The image guide fiber bundles 11 and 12 are placed in the fiber hearth nive 20, respectively, and the first optical low-pass filters F+ and Fl' are Inside the adapter 21, an image forming apparatus 13, a second optical low-pass filter F2, and a solid-state image pickup device 14 are arranged inside the TV camera 22, respectively.

FIG. 7 is a diagram showing the spatial frequency response on the image plane in this example on a two-dimensional frequency plane, where Pl and F8 are the spatial frequency bands of the image guide fiber bundle It and 12 cores, 15a and 15b, respectively. and 16a, 16
b is by each crystal plate 15 and 16 of the first optical low-pass filter F, , F,] Lap line, 17a, 1
7b, 18a, and 18b are trap lines formed by crystal plates 17 and 18 of the optical low-pass filter F2 of ff12.

The trap lines 15a and 15b are the spatial frequency band P of the core of the image guide fiber bundle 11, and the trap lines 15a and 15b are
ml, the trap &1t16a, 15t] is the spatial frequency band P2 of the core of the fiber bundle 12
It passes through the inner edge of Further, the trap line 17a17b passes through the current frequency 'HN+'NN.

Therefore, in this embodiment (according to the authors), a Noah Iherscope with a Si number of image guide fibers having different core spatial frequencies is used by selectively attaching it to the same TV camera with a fixed imaging magnification. In this case, moiré can be effectively removed without reducing resolution by selecting and using a first optical low-pass filter that corresponds to the spatial frequency.

Figure 8 shows the third embodiment, which consists of an image guide fiber bundle with a small effective diameter and an irregular fiber arrangement, and an image guide fiber bundle with a small effective diameter and a regular fiber arrangement. Two fiberscopes, each with a variable imaging magnification, can be selectively attached to the same I''VV camera and used. 31 has a small effective diameter and the fiber arrangement is irregular. A bundle of image guide fibers with a random diameter in the range of φ1 to φ1', 32 has a slightly larger effective diameter, a regular fiber arrangement, and a core diameter of φ.

is a constant image guide fiber bundle, 33 is an imaging lens which is a zoom lens, and 34 is a solid-state imaging device arranged on the imaging plane of the imaging lens 33. F is an optical low-pass filter disposed between the imaging lens 33 and the solid-state image sensor 34, and is composed of one or more crystal plates 35. The image guide fiber bundles 31 and 32 are placed inside the fiberscope 3G, respectively, and the imaging lens 33. Optical low-pass filter Fl solid-state image sensor 3
4 are respectively arranged in the TV camera 37.

Here, it is the number of pixels in the horizontal direction of the solid-state image sensor 34. If the imaging magnification of the imaging lens 33 is 8W (at wide-angle) to βT (at telephoto), then it is 2P.

φ, β.

φ1′ βa φtβ.

The image guide fiber bundle 31°32
If the core diameter and the imaging magnification of the imaging lens 33 are selected, beam separation in the horizontal direction becomes unnecessary, and beam separation only in the vertical direction can be performed using an optical low-pass filter F on the solid-state imaging device 34 side.

FIG. 9 is a diagram showing the spatial frequency response on the image plane in this example on a two-dimensional frequency plane, with Qw, Q
v is the spatial frequency band of the wide side and tele side cores of the image guide fiber bundle 31, Rw and Ry are the spatial frequency bands of the wide side and tele side cores of the image guide fiber bundle 32, respectively, and 35a and 35b are the optical frequencies When the low-pass filter F consists of a crystal plate 35 having a separation direction of 90° with respect to the horizontal axis, the trap lines 35C, 35d, 35e, and 35f due to the crystal plate 35 are the optical low-pass filter F shown in FIG. 1O. Two crystal plates 35 having a separation direction of 90° ±25° with respect to the horizontal axis as shown in FIG.
This is a trap line formed by the crystal plate 35 when the crystal plate 35 is composed of a 1/4λ plate 38 and a 1/4λ plate 38. Note that the 1/4λ heat wave 38 may be omitted, and the trap lines 35a, 35b, 35c 35
d5e 35f all pass through the spatial frequency band Qt Rw and become 2 2P □.

Therefore, according to this embodiment, the spatial frequencies on the image planes on the telephoto side when a small diameter image guide fiber bundle is used and on the wide side when a large diameter image guide fiber bundle is used are approximately equal. If the zoom ratio is determined as follows, and the imaging magnification is set so that these equal spatial frequencies are within the Nyquist frequency determined by the solid-state image sensor, a common optical low-pass filter can be applied to the solid-state image sensor side of the imaging optical system. By simply providing this, moiré can be effectively removed without reducing resolution.

(Effects of the Invention) As described above, the imaging device according to the present invention includes a plurality of fiberscopes having image guide fiber bundles with different effective diameters and core diameters, and the same imaging optical system having variable or fixed imaging magnification. This has a practically important advantage in that moiré can be prevented from occurring without impairing resolution when viewing on a TV monitor with a camera selectively attached.

It also has the advantage of being effective even when the fiber arrangement of the image guide fiber bundle is irregular.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram of the optical system of the first embodiment of the imaging device according to the present invention, Fig. 2 is a diagram showing the spatial frequency response of one crystal plate, and Fig. 3 is a diagram showing the spatial frequency response of one crystal plate. FIG. 4 is an exploded perspective view of the first optical low-pass filter of the first embodiment. FIG. 5 is a schematic diagram of the optical system of the second embodiment. Fig. 6 is an exploded perspective view of the second optical low-pass filter of the second embodiment, Fig. 7 is a diagram showing the spatial frequency response on the image plane in the second embodiment on a two-dimensional frequency plane, and Fig. 8 is an exploded perspective view of the second optical low-pass filter of the second embodiment. A schematic diagram of the optical system of the third embodiment, FIG. 9 is a diagram showing the spatial frequency response on the image plane in the third embodiment on a two-dimensional frequency plane, and FIG. 10 is an optical low-pass filter of the third embodiment. The exploded perspective views of FIGS. 11 to 13 are diagrams showing the relationship between the Nyquist frequency of the solid-state imaging element and the spatial frequency of the core of the image of the east end of the image guide fiber. 1, II, 12, 31.32... Image guide fiber bundle, 2.13.33... Imaging lens, 3.1
4.34...solid-state ti image element, 4,5,6°15,
+6.17, 18.35...Crystal plate, FF1'.
...First optical low-pass filter, F2...Second optical low-pass filter, F...Optical low-pass filter, ? , 20.36...Scope, 8,2
2.37...TV camera, 9,19°38...
1/4λ plate, 21...adapter. Figure 1 Figure 2 Figure 5/22 Figure 7 Figures 1-10, Contents of amendment (1) N63416J on the bottom 3rd page line 7 of the specification has been changed to f193
Corrected to 416J. (2) [Figures 10 to 12] on page 5, line 9
Figures 1 to 13 are corrected. (3) "Figures 1O and 11" in the 6th line of page 6 are corrected to rFigures 11 and 121. (4) "Figure 1O" on page 6, line 8 and page 7, line 2 are corrected to "Figure 11 1." (5) Page 6, line 20, page 8, line 6 1'', page 8, line 7, line 11, ``Figure 11'' is corrected to 1, Figure 12, 1, respectively (spontaneous) Patent Application No. 1983, No. 290024, No. 5-19, Shinbashi, Minato-ku, Tokyo, 105 Specification A column for a detailed description of the invention.

Claims (1)

[Claims] An objective lens that forms an image of an object, an image guide fiber bundle that transmits the object image by the objective lens, and an image guide fiber bundle that transmits the object image appearing at the exit end face of the image guide fiber bundle onto a solid-state image sensor. An imaging device comprising an imaging lens that re-images, a first optical low-pass filter having a spatial frequency response that corresponds to the fiber arrangement of the image guide fiber bundle, and a first optical low-pass filter that has a spatial frequency response that corresponds to the sampling frequency of the solid-state image sensor. An imaging device characterized in that a second optical low-pass filter having a spatial frequency response is provided in an imaging optical path from an exit end face of the image guide fiber bundle to a solid-state imaging device.